If a voltage higher than a reference voltage of a unit battery (cell) is needed such as in a hybrid vehicle which uses a lithium ion cell as a power source, it is typical to connect a plurality of unit batteries in series. However, even though the batteries produced via the typical manufacturing method have the same structure using the same anode, cathode and electrolyte, there is existed a difference in charge or discharge characteristics between each battery connected in series.
Therefore, since a voltage difference exists between the unit batteries when using the batteries connected in series, there are problems in that a total voltage (total voltage of batteries connected in series) becomes zero so that a recharge is needed even though one battery is totally discharged irrespective of voltage of the other batteries in the unit batteries connected in series, the battery reaching the certain voltage at first is overcharged due to different voltage between each battery even at the time of the recharge, and the battery which still does not reach any certain voltage exists even though some batteries are overcharged.
Further, if the number of charging/discharging times is higher, significant degradation can occur in materials composing the battery so that characteristics of the battery become different and such degradation is responsible for aggravating the difference between the individual cells.
Therefore, in order to address such problems, various charge equalization apparatuses are actively proposed to achieve charge equalization of the batteries connected in series.
As an example, Korean Patent Laid-Open No. 2006-0078967 is directed to a system which is consisted of a serial battery pack; two or more battery modules composing the serial battery pack; a monitoring module sensing a voltage of the battery module; a switch module; a control module controlling the monitoring module and the switch module, to bypass the battery module of which the current reaches a certain voltage value by controlling the switch module via the control module if the voltage of the battery module reaches the certain voltage upon charging the battery pack. Korean Patent Laid-Open No. 2003-0096978 is directed to a system which is consisted of a plurality of unit cells, a charging means, a discharging means, and serial-parallel conversion switch, and performs charging by discharging the plurality of unit cells equally and then connecting the discharged unit cells in series using the serial-parallel conversion switch. Korean Patent Laid-Open 2007-0064244 is directed to a system which comprises a battery unit, a field-effect transistor unit connected to the battery unit, an amplifying unit connected to the field-effect transistor unit, a multiplexer controlling an output signal of the amplifying unit, a comparator comparing and determining a variation in a voltage signal of the battery unit, an A/D converting unit converting the output from the comparator into a digital signal, a Micom unit inputted with the signal outputted from the A/D converter and outputting a signal corresponding to the charging/discharging condition, a switching unit supplying the battery equalization current in accordance with the signal of the Micom unit, and a known charging/discharging circuit.
Further, Japanese Patent Laid Open No. 2008-220110 is directed to a system consisted of a secondary battery having a plurality of battery cells connected; a switch element controlling charge/discharge of the secondary battery; a measuring unit measuring each voltage of the battery cells and detecting a maximum voltage of the battery cell; a switch element control unit controlling the switch element on a basis of the detected voltage of the measuring unit; a storage means storing charge completion voltage and maximum charge current value; a charge current specific-value control unit changing the charge current value in accordance with the detection voltage of the battery cell. Japanese Patent Laid Open No. 2008-199798 is directed to a system consisted of a serial-connected battery block group; a discharging circuit group connected in serial with each other and connected in parallel to each of the battery blocks by the switch group; a charging circuit group connected in parallel to the discharging circuit while being connected in serial with each other and connected in parallel to each of the battery blocks by the switch group; a charging/discharging control unit controlling the discharging circuit group, the charging circuit group, and the switch group. Japanese Patent Laid Open No. 1998-032936 is directed to a system which is consisted of a plurality of unit cells, a detection means for detecting remaining capacity in each unit cell, a charge replacement means and a discharge replacement means for performing charging and discharging on each unit cell, a control means for controlling charging and discharging in each unit cell separately, and a direct current/direct current converter performing charging and discharging separately on each unit cell. Japanese patent Laid Open No. 2004-194410 is directed to a system which is consisted of two or more unit cell groups, a current difference detection means for detecting a difference in currents flowing in each of a first cell group and a second cell group, and a means for controlling charging/discharging current in the cell group based on the difference in currents.
U.S. Patent Laid-Open No. 2007-0222416 is directed to a system which is consisted of a backflow prevention switch connected between a battery and a ground; a charging switch; a serial circuit of a resistor for detecting the current; a charging control circuit controlling the backflow prevention switch and the charging switch and detecting a difference voltage between a regulated voltage of the battery and a open voltage of the battery while repeating charging and opening of the battery; and a regulated current-regulated voltage control circuit controlling the charging regulated voltage based on the difference voltage detected.
However, since the above-mentioned charge equalization apparatuses have a charge equalization apparatus provided in each of the batteries connected in series so that charging or discharging is performed separately on each battery and each voltage of the batteries is measured and converted to the digital value separately to determine whether or not to charge or discharge the battery and to set charge equalization conditions based on the voltage of the battery, there are problems that complexity and volume of the charge equalization apparatus are increased and thus productivity is reduced and production cost is higher, and components composing the charge equalization apparatus or the switch module for controlling it must endure higher voltage stress.